Laser power modulation to grain refinement of SAE 1045 steel welds

Refined grain size structure is one of the most sought-after features during parameterization of welding processing. Refined grains in the region of the molten zone provide high tenacity in addition to high hardness, due to the dislocation blocking by grain boundaries. However, obtaining refined grain structure is not trivial, since a molten pool is formed during bonding of materials via welding, which often results in grain growth. In this sense, this study aimed to refine the granular structure of an SAE 1045 steel by modulating laser power. The success through the modulation method in some aspects of welding is well known from the literature, such as the reduction of pores and cracks as well as deep penetration with narrow heat affected zones. These enhancements are due to cooling rate control produced by the nonuniform way of interaction between the laser and the specimen. In the present study, 1045 steel specimens were first welded utilizing the conventional continuous mode and then the results were compared with a modulated rectangular-shaped power mode while maintaining the average power constant. Microhardness measurements were used in order to investigate the mechanical effect of grain size changes. It was found that an increase in hardness of 50 HV was achieved by modulating the laser power in time. This increase is credited to the reduction in grain size of the studied steel. It can be assumed that the time modulation of the laser power provides a better control of the cooling rate and consequently the mass flux in the molten pool, what may explain the hardening effect by grain refinement.